The present invention generally relates to the technology of optical telecommunication and more particularly to an optical semiconductor device such as a laser diode used in optical telecommunication systems. Especially, the present invention is related to a surface-emission laser diode and fabrication process thereof, a growth apparatus and a growth process used for forming such a surface-emission laser diode, as well as optical transmission modules, optical transceiver modules and optical telecommunication systems that use such a surface-emission laser diode.
With wide spread use of Internet, there is going on some kind of explosion of information handled by telecommunication systems. In view of this situation, optical fibers are now being deployed not only in trunk lines but also in subscriber lines or LANs (local area networks) located near the side of users. Further, optical fibers are introduced also for interconnection of various apparatuses or for interconnection inside an apparatus. Thus, the importance of large-capacity optical telecommunication technology is increasing evermore.
In order to realize inexpensive long-range optical telecommunication networks or large-capacity optical telecommunication networks, it is advantageous to use a vertical-cavity surface-emission laser diode (VCSEL, referred to hereinafter simply as “surface-emission laser diode”), in which an optical cavity is provided in a direction perpendicular to the epitaxial layers constituting the laser diode. In view of minimum optical loss of silica-based optical fibers in the wavelength band of 1.3 μm and 1.55 μm, the surface-emission layer diode for use in such an optical telecommunication system is required to oscillate at the wavelength band of 1.3–1.55 μm. Here, the use of a surface-emission laser diode is particularly advantageous in view of its low cost, low power consumption, compact size and easiness of forming a two dimensional array.
There already exists a surface-emission laser diode constructed on a GaAs substrate and operable in the wavelength band of 0.85 μm. Thus, such a surface-emission laser diode is used in a high-speed LAN such as 1 Gbit/s Ethernet.
In the 1.3 μm band, on the other hand, a semiconductor material of InP has been used commonly in the conventional edge-emission type laser diodes. On the other hand, conventional laser diode of the 1.3 μm band thus constructed on the InP substrate has suffered from the problem of large increase of drive current, by the factor of three times or more, when the environmental temperature has increased from room temperature to 80° C. Further, there has been a problem in that no satisfactory distributed Bragg reflector could be constructed on an InP substrate, and thus, it has been difficult to construct a surface-emission on such an InP substrate.
In view of the foregoing difficulty, there has been a proposal to bond an active structure of a surface-emission laser diode including an InP substrate and an active layer on a distributed Bragg reflector of an AlGaAs/GaAs stacked structure formed on a GaAs substrate (V. Jayaraman, J. C. Geske, M. H. MacDougal, F. H. Peters, T. D. Lowes, and T. T. Char, Electron. Lett., 34, (14), pp.1405–1406, 1998).
However, such a construction becomes inevitably expensive and has an obvious problem of poor efficiency of production.
In view of the foregoing problems, efforts are being made to construct a surface emission laser diode operable in the wavelength band of 1.3 μm on a GaAs substrate, by using (Ga)InAs quantum dots for the active layer, or a compound semiconductor material such as GaAsSb or GaInNAs for the active layer. Reference should be made to Japanese Laid-Open Patent Publication 6-37355. Particularly, GaInNAs is expected as being a semiconductor material capable of minimizing the temperature dependence of the laser diode.
A GaInNAs laser diode constructed on a GaAs substrate has an advantageous feature of reduced bandgap for the active layer as a result of incorporation of N in the active layer, and thus, the laser diode becomes operable in the wavelength band of 1.3 μm even in the case the laser diode is constructed on a GaAs substrate. When the In content is 10%, for example, the wavelength band of 1.3 μm is realized by introducing N into the active layer with a concentration of about 3%.
FIG. 1 shows the relationship between the threshold current density of laser oscillation and the N content in the active layer for a laser diode having a GaInNAs active layer, wherein the vertical axis represents the threshold current density while the horizontal axis represents the N content in terms of percent.
Referring to FIG. 1, it can be seen that there occurs a steep increase in the threshold current with the N content in the active layer, wherein it is believed that the relationship of FIG. 1 reflects the situation in which the degree of crystallization of the GaInNAs active layer is deteriorated with increase of the N content in the active layer.
Thus, the growth process of high quality GaInNAs layer becomes the key issue in the fabrication of such a surface-emission laser diode operable at the wavelength band of 1.3–1.55 μm.
Generally, a GaInNAs layer can been grown by an MOCVD (metal organic chemical vapor deposition) process or MBE (molecular beam epitaxy) process, wherein MOCVD process, in which the supply of source material is controlled by controlling a gas flow rate of the source material, is thought more advantageous and suitable for mass production of the laser diode as compared with MBE process, in which the supply of the source material is controlled solely by the control of the temperature of source cells, in view of the fact that the MOCVD process does not require a highly vacuum environment such as the one needed in the case of an MBE process, and a large growth rate is achievable easily. Thereby, the throughput of device production can be increased. In fact, mass production of the surface-emission laser diodes of the 0.85 μm band is achieved already by using an MOCVD process.
FIG. 2 shows the construction of a typical MOCVD apparatus used for growing group III–V semiconductor layers.
Referring to FIG. 2, the MOCVD apparatus is generally formed of a source gas supply system A for supplying a source gas, a susceptor B for supporting the substrate S and an evacuation unit C such as a vacuum pump for evacuating gases that have caused a reaction.
Generally, the substrate S is first loaded in a load/unload chamber 11 and is then transported to a growth chamber (reaction chamber) 12 after evacuating the air in the load/unload chamber 11 by driving the evacuation unit C.
Typically, the growth chamber is controlled to have an internal pressure of 50–100 Torr, and one or more of the metal organic sources such as TMG (trimethyl gallium), TEG (trimethyl gallium), TMA (trimethyl aluminum), TMI (trimethyl indium), and the like, are introduced into the growth chamber 12 as the source of group III element, together with a source of group V element. As a group V source, a hydride gas or organic compound such as AsH3, TBA (tertiary butyl arsine), PH3, TBP (tertiary butyl arsine), and the like is used.
These gaseous sources are transported to the growth chamber by a hydrogen carrier gas, wherein the hydrogen carrier gas is generally used after removing impurity by passing through a purifier 13.
For the source of nitrogen, organic compounds such as DMHy (dimethyl hydrazine), MMHy (monomethyl hydrazine), and the like, are used, although there are possibilities of using other materials.
In the case of using a liquid or solid source material, the source material is held in a bubbler 14 and the vapor of the source material or source gas formed as a result of bubbling in the bubbler 14 by the carrier gas, is supplied. The hydride gas, on the other hand, is held in a gas cylinder 15.
In the example of FIG. 2, two bubblers #1 and #2 are provided for holding two difference source materials and two gas cylinders #1 and #2 are provided for holding two different gaseous source materials.
The path of the source gases is selected according to the needs by controlling the valves forming a valve array 16, and the flow rate of the individual gases is controlled by using a mass flow controller (MFC). In such a gas supply system, it is practiced to provide dummy gas lines such as lines #1 and #2 for avoiding change of gas flow rate, gas pressure, and the like.
In an MOCVD process that is conducted by using the system such as the one shown in FIG. 1, the thickness of the semiconductor layers is controlled by way of controlling the duration of supply of the source gases. Thus, the MOCVD process provides excellent throughput and is thought most suitable for mass production of semiconductor devices.
FIG. 3 shows a typical MBE apparatus.
MBE process is a modification of the vacuum evaporation deposition process and uses source molecules or atoms emitted from a source cell. The source molecules or atoms thus emitted by the source cell cause a deposition on the surface of a heated substrate after traveling through a growth chamber, which is held in a high vacuum state.
Referring to FIG. 3, the MBE apparatus includes a growth chamber 21 coupled with the load/unload chamber 11 similarly to the MOCVD apparatus of FIG. 2, wherein the growth chamber 21 is evacuated to a high vacuum state by the vacuum pump 23 and the substrate S is held on a susceptor 25 having a heating mechanism. The growth chamber 21 is provided with molecular beam cells 21A and 21B for holding solid sources and also a molecular beam cell 21C of nitrogen, which is actually a nitrogen radical cell.
As no hydrogen or carbon is contained in the source in the case of the MBE process, the semiconductor layers grown on the substrate 24 contains little impurities and high quality semiconductor layers are obtained.
On the other hand, the MBE process has a drawback in view of the need of high vacuum state in that it is not possible to increase the supply rate of the source materials. When the supply rate of the source materials is increased, the load of the evacuation system becomes excessively large, and the system would undergo frequent failure or need frequent maintenance operations. Thus, the MBE process inherently suffers from low throughput.
While there are reports in these days about laser diodes, including surface-emission laser diodes, that use the system of GaInNAs for the active layer, that are operable in the wavelength band of 1.2 μm (M. C. Larson, et al., IEEE Photonics Technol. Lett., 10, pp.188–190), most of the reports are based on the devices produced by an MBE process.
In view of the poor throughput of the MBE process noted above, and further in view of the fact that the GaInNAs surface-emission laser diode thus formed by the MBE process suffers from the problem of very large resistance caused by the p-side distributed Bragg reflector, the inventors of the present invention have conducted a series of experimental investigations on the fabrication process of GaInNAs laser diode, particularly a surface-emission laser diode that uses GaInNAs formed by an MOCVD process for the active layer and identified the cause of deterioration of the GaInNAs active layer crystal.
More specifically, the Japanese Laid-Open Patent Publication 10-126004 describes the discovery made by the inventors of the present invention in that there occurs a segregation of N at the interface between a GaInNAs active layer and an underlying Al-containing layer when the GaInNAs active layer is grown in direct contact with the underlying Al-containing layer Al and that such a segregation of N causes a substantial deterioration of surface morphology of the GaInNAs active layer. Thus, in order to avoid the foregoing problem, the foregoing Japanese Laid-Open Patent Publication 10-126004 has proposed a structure in which there is provided a layer free from Al such that the Al-free layer makes a direct contact with the GaInNAs active layer.
Further, Japanese Laid-Open Patent Publication 2000-4068 proposes a structure that provides an intermediate layer free from both Al and N between the GaInNAs active layer and the AlGaInP cladding layer for avoiding the degradation of the GaInNAs active layer.
On the other hand, there is a report that there can be caused a degradation of efficiency of optical emission in the GaInNAs active layer provided on an Al-containing semiconductor layer even in such a case in which an intermediate layer free from Al and N is interposed between the Al-containing semiconductor layer and the GaInNAs active layer. For example, there is a report in Electron. Lett., 2000, 36 (21), pp.1776–1777 reports the discovery of severe degradation of photoluminescence intensity in a GaInNAs quantum well layer grown by an MOCVD process on an AlGaAs cladding layer continuously. In order to improve the photoluminescence intensity, the foregoing literature thus uses different MOCVD chambers for growing the AlGaAs cladding layer and the GaInNAs active layer.
FIG. 4 shows the room-temperature photoluminescent spectrum of a GaInNAs/GaAs double quantum well structure formed by the inventor of the present invention by an MOCVD process, wherein it should be noted that the curve designated as “A” represents the case in which the GaInNAs/GaAs double quantum well structure is formed on an AlGaAs cladding layer with an intervening GaAs layer, while the curve designated as “B” represents the case in which the GaInNAs/GaAs double quantum well structure is formed on a GaInP cladding layer with an intervening GaAs layer.
As can be seen in FIG. 4, the photoluminescence intensity for the sample A is less than one-half of the photoluminescent intensity for the sample B.
The result of FIG. 4 thus shows clearly that there occurs severe degradation in the efficiency of optical emission in the GaInNAs layer in the case the GaInNAs layer is formed on a semiconductor layer containing Al as a constituent element such as AlGaAs by using a single MOCVD apparatus, even in the case an intervening Al-free intermediate layer is provided. Associated with such a degradation of quality of the GaInNAs layer, the laser diode that uses such a GaInNAs layer suffers from the problem of poor threshold characteristics characterized by a large threshold current, which can become twice as large as the threshold current for the case in which the same active layer is formed on a GaInP cladding layer.